Process for producing insoluble sodium metaphosphate



Patented Aug. 29, 1944 raoqns s FOB rnonucmdmsownm SODIUM mz'rarnosrm'rnGeorge E.

Ala., asslgnors Taylor and Allen G. Erdrnan to Monsanto Chemical Cam'-Anniston,

pany, a corporation Delaware No Drawing. Application May 27, 1943,Serial No. 488,776

8 Claims.

This invention relates to a process for producing insoluble sodiummetaphosphate.

An. object of this invention is to provide a process whereby insolublesodium metaphosphate may be produced without the simultaneous productionof excessive amounts of soluble forms of sodium metaphosphate. Y

The use of the insoluble sodium metaphosphate as a dentifrice was firstdisclosed by Kuever in U. S. 2,019,142. Y

In the commercial manufacture of insoluble sodium metaphosphate whichmanufacture is carried out by heating mono-sodium orthophosphate totemperatures in the range of from 400 C. to 460 C., it has been foundthat relatively large amounts of the soluble metaphosphates aresimultaneously formed. Since insoluble sodium metaphosphate produces anideal polishing ingredient either for tooth powders or tooth pastes andmoreover since the soluble variety of sodium metaphosphate has beenfound to have a deleterious effect upon tooth enamel, it is highlydesirable that the insoluble sodium metaphosphate be produced without,at the same time producing the soluble variety.

While it is possible to overcome the effect of minor proportions ofsoluble sodium metaphosphate by adding various alkaline or alkalineearth compounds such as magnesium or calcium salts to the insolublesodium metaphosphate when formulating a dentifrice composition, assuggested by Long in U. S. Patent No. 2,216,821, nevertheless thesoluble variety of sodium metaphosphate in the insoluble sodiummetaphosphate represents a deleterious constituent which must either becounteracted or must be removed from the composition as by leaching,etc.

Previous methods of producing insoluble sodium metaphosphate haveconsisted in heating either the crystalline or the finely groundmono-sodium orthophosphate to metaphosphate forming temperatures, theheating being carried out in trays or pans. Such heating was necessarilycarried out as a batch operation, the heating period being of the orderof 14 hours.

I have now found that if finely ground crystalline mono-sodiumorthophosphate in the anhydrousform be first formed into pellets bycompression, that the pellets thus formed may be heated to themetaphosphate forming temperature in a rotary kiln or shaft furnace orother heating device and a good yield of insoluble sodium metaphosphateobtained which is surprisingly free of soluble metaphosphates.

A satisfactory pellet of mono-sodium orthophosphate is made by merelybriquetting the orthophosphate powder as such or the powder may first besomewhat moistened utilizing up to 5% by weight of water as the bindingagent. after which the slightly moist orthophosphate is then briquetted.I have found that from 1% to 2% of water is usually suflicient formaking a strong briquette from the powdered orthophosphate. Ifdesirable, the water may also be added by adding the equivalent amountof the monoor dihydrate of mono-sodium orthophospnate.

The strength of the briquette should be such that it will maintain itsshape during the subsequent treatment wherein the briquette is heated toconvert the orthophosphate to the insoluble metaphosphate. The size ofthe compressed pellet produced is not particularly critical except thatthe pellet should be small enough so that it may be thoroughly heated tothe metaphosphate conversion temperature by circulating hot gases in arelatively short time. We have found that a circular pellet up to 1 inchdiameter and from to in thickness may conveniently be employed. On theother hand, if desired, the pellet size may be decreased to that sizenow used for aspirin tablets. In general, pellets weighing between 0.5gram and 25 grams are preferred and may be used satisfactorily. Theanhydrous mono-sodium orthophosphate employed for producing the pelletsmay vary in particle size over a considerable range but we have foundthat 40 mesh is close to the upper limit of particle size to give asatisfactory product. Material which passes a mesh screen is definitelypreferable and particle sizes as fine as 350 or 400 mesh may be usedquite satisfactorily.

Pellets produced as described above may now be heated in a rotary kilnor converter in the well known manner. A suitable device consists of arotary kiln provided with a gas burner located at the discharge endthereof and supplied at the entrance to the kiln with hot gas at atemperature of about 500 C. The pellets of orthophosphateare introducedat the inlet or upper end of the inclined rotary kiln and due to theinclination and rotation of the kiln are caused to move downwardlythrough the kiln countercurrently to the heated gases. The pellets uponencountering gas of this temperature are heated gradually to atemperature which should not be higher than about 450 C. to 460 C.Conversion to the insoluble metaphosphate takes place between the limitsof from 300 C. to 460 C. The inclination of the axis of the kiln shouldbe such that the entire heating of the stream of pellets takes place infrom 1 to 1% hours or possibly 2 hours. At all events the heating shouldbe completed in less than 3% or 4 hours.

Alternatively, the pellets may be heated by passing-them counter-currenttoa flow of hot gases, through a vertical shaft kiln. The pellets arepassed through such a kiln by removing the heated and converted productfrom the bottom of the kiln, 'while adding fresh pellets at the top ofthe kiln. The time of heating in such a device is thus governed by therate otdlscharge from the lower portionof the kiln and may be adjustedto conform with the time or heatin obtained in the rotary kiln, as givenabove.

The hot pellets discharged from thekiln are allowed to cool to roomtemperature and then are crushed and finely ground to produce a powderfine enough to pass a 350 mesh screen.

Using the herein described process we have found that the water solublesalt content of the product is maintained under 4% and generally under2%. tent will be found to be below a fraction of 1%.

What we claim is: v

1. The method for producing insoluble sodium metaphosphate comprisingcompressing finely monosodium orthophosphate into pellets, heating saidpellets to a temperature favorable to the formation of insoluble sodiummetaphosphate and grinding said pellets to a fine powder.

2. The method for producing insoluble sodium metaphosphate comprisingcompressing finely divided monosodium orthophosphate into pellets.heating said pelletsto a temperature between 300 C. and 460 C. to'forminsoluble sodium In many cases the soluble salt coni metaphosphate andgrinding said pellets to a fine powder.

3. The method for producing the insoluble metaphosphate of sodium whichcomprises forming pellets by compressin a mixture of monosodiumorthophosphate and water in an amount less than 5% and then heating saidpellets in a stream of air to a temperature between 300 C. and 460 C. toform insoluble sodium metaphosphate, and then grinding said pellets to afine powder.

4. The process defined in claim 3 in which the time of heating isbetween one hour and four hours.

5. The process defined in claim 3 in which the pellets flowcounter-current to the stream oi air.

6. The process defined in claim 3 in which the particle size of themono-sodium phosphate before pelleting is fined than mesh.

7. The process defined in claim 3 in which the particle size of themonosodium phosphate before pelleting is finer than mesh.

8. The method for producing the insoluble metaphosphate of sodium, whichcomprises forming pellets from finely divided monosodium orthophosphate,said orthophosphate having a screen size of less than 40 mesh and saidpellets weighing between 0.5 gram and 25 grams each, heating saidpellets in a stream of air to a temperature between 300 C. and 460 C.for a period of from 1 to 4 hours and then crushing said pellets to afine powder.

GEORGE E. TAYLOR. ALLEN G. ERDMAN.-

3 i CERTIFICATE OF CORRECTION? M Patent No. 2,556,799. H Auguet. 29.,191 1;;

. GEORGE E. TAYLOR, ET AL.

1111s hereby certified that error appears in the printed specification.of the above numbered patent requiring correctionaefoll'ows: Page first column, 1111.6"27 claim 1, before the word 'fmonosodium insert'--d1*g idedand second'coltmmiline 19, claim 6, for "fined" read-.-.-f1nera nd that the said Letters Patent should be read; with 1111scorrectien therein that the same may conform be the record of the casein the Patent-qf f icmfl S1gned and seeled this 21pm day of October, A.D. 191 1;.

Leelie Frazer .(seal) 3 Acting Ccnnniseicner 01E Patents.

